There are TCP ports available for reverse
telnet connectios to physical lines on
a NAS (usually); so port 2001 would be
for line 1 of the NAS.
You can disable this entirely with the
line command .
Or, you can use AAA:
http://www.cisco.com/univercd/cc/td/doc/product/software/ios121/121cgcr/secur_c/scprt1/scdathor.htm#xtocid1433612
The ports that are potentially usable are here:
http://www.cisco.com/univercd/cc/td/doc/cisintwk/itg_v1/tr1916.htm#xtocid203153